Add flag for disallowing for-in initializers

src/parsing/parser.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/parsing/parser.h"
 
 #include "src/api.h"
 #include "src/ast/ast.h"
 #include "src/ast/ast-expression-rewriter.h"
 #include "src/ast/ast-expression-visitor.h"
@@ skipping 3482 matching lines @@
           *ok = false;
           return nullptr;
         }
 
         Block* init_block = nullptr;
 
         // special case for legacy for (var/const x =.... in)
         if (!IsLexicalVariableMode(parsing_result.descriptor.mode) &&
             decl.pattern->IsVariableProxy() && decl.initializer != nullptr) {
           ++use_counts_[v8::Isolate::kForInInitializer];
+          if (FLAG_harmony_for_in) {
+            // TODO(rossberg): This error is not currently generated in the
+            // preparser, because that would lose some of the use counts
+            // recorded above. Once either the use counter or the flag is
+            // removed, the preparser should be adjusted.
+            ParserTraits::ReportMessageAt(
+                parsing_result.first_initializer_loc,
+                MessageTemplate::kForInOfLoopInitializer,
+                ForEachStatement::VisitModeString(mode));
+            *ok = false;
+            return nullptr;
+          }
           const AstRawString* name =
               decl.pattern->AsVariableProxy()->raw_name();
           VariableProxy* single_var = scope_->NewUnresolved(
               factory(), name, Variable::NORMAL, each_beg_pos, each_end_pos);
           init_block = factory()->NewBlock(
               nullptr, 2, true, parsing_result.descriptor.declaration_pos);
           init_block->statements()->Add(
               factory()->NewExpressionStatement(
                   factory()->NewAssignment(Token::ASSIGN, single_var,
                                            decl.initializer,
@@ skipping 3270 matching lines @@
                         try_block, target);
     final_loop = target;
   }
 
   return final_loop;
 }
 
 
 }  // namespace internal
 }  // namespace v8